Differential signals are frequently employed in high noise environments. A differential receiver can receive a signal on two wires. The signal can be comprised of a common-mode signal with a differential signal superimposed on the common mode signal. Stripping away the common mode signal yields the differential signal. For example, the Controller Area Network (CAN) standard employs a differential signal superimposed on a common mode signal. The CAN standard defines a bus (CAN bus) that is a two-wire, half duplex, high-speed network system that is well suited for high speed applications using short messages. The CAN bus is frequently used in embedded systems having a plurality of microcontrollers.
The current CAN specification is divided into two parts, Standard CAN (version 2.0A) and Extended CAN (version 2.0B). The two versions define different formats of the message frame, with the main difference being the identifier length. Standard CAN uses 11 bit identifiers, while Extended CAN uses 29 bit identifiers. There are two ISO standards for CAN, ISO 11519 and ISO 11898. The difference between the two standards is in the physical layer, where ISO 11519 handles low speed applications up to 125 Kbyte/second and ISO 11898 handles high speed applications up to 1 MB/second.
The CAN bus consists of two wires, a CAN-High (CANH) wire and CAN-Low (CANL) wire. These two wires operate in differential mode as they are carrying inverted differential voltages superimposed on a common-mode signal to decrease noise interference. Currently available CAN transceivers, whether using off-the-shelf components or integrated into an application specific integrated circuits (ASICs), use a voltage based reception of the differential signal on the CAN buses, which is received on the CANH and CANL wires.
The CAN standard imposes a high input common mode voltage range (−30 V to 30 V) that exceeds the typical CAN supply voltage (5V) of CAN receivers. Currently available receivers attenuate both the signal and the common mode range to bring the signal to the 0-5V supply limited common-mode range before the signal gets resolved by the receiver. Attenuating a signal before receiving it is disadvantageous as it degrades the quality of the signal, thus increasing the susceptibility to noise and probability of reception errors.
For example, the CAN standard can employ a differential voltage of 900 mV or greater to indicate a logic “1” state, and a differential voltage of 500 mV or less to indicate a logic “0” state. If the differential voltage is between 500 mV and 900 mV, then similar to a hysteresis effect, the previous logic value is retained. As an example, a signal on CANH can be 30.5 V, while the signal on CANL is 29.5 V, or a signal on CANH can be −29.5 V, while the signal on CANL is −30.5 V (CANH is always greater than CANL) for a differential of one volt, which should be interpreted as a logic “1”. Reducing the bus voltage to be within the range of the receivers supply voltage (5V) would also reduce the differential voltage to ⅙ of its original voltage increasing the possibility of errors in interpreting the logic levels associated with the differential signal. Amplifiers can be used to amplify the reduced differential voltage to its original signal level at the receiver, however amplifiers are susceptible to noise and can introduce additional undesirable errors, especially in noisy environments.